I too think the rear axle it's self is not a weak link, it's the math and the gear ratio that will hold you back.

Here's a gear calculator for you to play with
http://www.onlineconversion.com/bigger_tires.htm

In my experience the 55 model 235 in my 1 ton ran well at 2,500 rpm or thereabouts, anything over that just wasted fuel, it would run faster but didn't seem as happy.

So what transmission (overdrive) ratio, axle ratio, tire size, and engine RPM do you need to go the speed you want? And then will the engine actually pull that?

I feel that my 1 ton would be great with 4.10 gears, 33.5" tires, the original 4 speed, and the newer 235, but any faster gears and the engine lacks power.
Most recently I ran 4.56, which is great for a working truck on short trips, 50-55 mph but empty trips over an hour and the one step faster gears would be welcome.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-